The utility of battery operated, emergency light sources for industrial and office buildings is well established. Typically such devices are permanently connected to the line voltage normally available within the plant or office building and include a relatively powerful lamp for energization by a heavy, bulky storage battery. The battery for example may be a series of wet cells and the device may provide for automatically maintaining the proper charge on the battery.
Although this equipment is probably suitable for its particular application, the large size, high cost and permanent mounting of these known emergency light sources renders them impractical for home use. Nevertheless, it would be desirable to provide a similar, although more versatile emergency light for use in the home, small shop or office, and similar circumstances where low cost, small or compact size, and portability are functional and desirable features. The value of such a home emergency light is particularly apparent for communities or regions where power "black-outs" can or are likely to occur.
Another disadvantage of existing emergency lighting equipment lies in their complex and many times unreliable control circuitry for sensing the loss of power and connecting the lamp across the storage battery in response thereto. For example, many known devices employ moving parts such as electromechanical relays and switches. Such components may malfunction after sitting unused for many months or years. Furthermore, the relatively high component cost of such electromechanical devices, when incorporated into the control circuit for the light source, preclude any substantial reduction in the overall cost of the unit.
Accordingly, it is one object of the present invention to provide a low cost, convenient emergency lamp adapted for connection to a normally available source of power, such as household line voltage, wherein the light source is automatically energized in the event of a power loss.
A further object of the present invention is to provide such an emergency lamp in the form of a small, compact portable unit, similar to a flashlight, which may be detachably plugged into a standard household utility outlet or receptacle.
Another object of the present invention is to provide an emergency lamp of the characteristics above indicated, having a rechargeable storage battery which may be continuously charged by the normally available power source, such that the battery is always maintained in a condition for emergency use.
Still a further object of the present invention is to provide a relatively simple, extremely low cost, reliable solid state switching circuit for automatically charging a re-chargeable battery and simultaneously maintaining the lamp unenergized in response to the continued presence of an external source of power, e.g., household or line voltage, and for energizing the lamp from the battery in the event of loss of such external power.
Additionally, it is an object of the present invention to provide a portable, emergency lamp having the foregoing characteristics in which the lamp is selectively deenergized by detachable connection to a source of external power and is energized by detaching the unit from such external power, such that no electromechanical switch is required.
Still another object of the present invention is to provide an extremely reliable, low cost, solid state switching circuit for use in controlling the emergency lamp disclosed herein or for general switching applications.
A battery operated emergency lamp is provided for connection to a normally "on" electrical source of power, such as a standard ac line voltage, where such source is subject to occasional power failures. A solid state switching circuit including a single transistor and one diode serves to maintain the lamp in an unenergized condition and to simultaneously generate a low charging current for a rechargeable battery so long as the power source remains "on." In the event of a power interruption, the solid state switching circuit is immediately responsive to switch to a condition in which the lamp is energized by a battery current. In one embodiment disclosed herein, the emergency lamp takes the form of a relatively small, portable lantern or flashlight equipped with a standard utility plug for connection to a household electrical outlet. With the lamp plugged into the wall receptacle, the light source is maintained in an unenergized state while the rechargeable battery is continuously charged for availability in the event of an emergency blackout or other power loss. When the power failure does occur, the lamp is automatically turned "on" to provide emergency light and to indicate the position of the device for removal from the wall receptacle and use as a portable flashlight. Additionally, the light may be employed as a convenient, always ready, portable lantern in which its removal from the electrical receptacle automatically energizes the light.
Other embodiments of the invention are also provided. For example, the external power source subject to failure, may be either the alternating current line voltage referred to above, or a direct current source such as provided by a dc line voltage on marine craft.
Although one of the embodiments of the invention has the feature of not requiring any electromechanical or other manually operated switch for turning the lamp on or off, an alternative embodiment disclosed herein provides such a switch to afford certain auxiliary lighting modes.